U.S. patent number 8,745,004 [Application Number 13/168,479] was granted by the patent office on 2014-06-03 for reverting an old snapshot on a production volume without a full sweep.
This patent grant is currently assigned to EMC Corporation. The grantee listed for this patent is Benny Assouline, Assaf Natanzon. Invention is credited to Benny Assouline, Assaf Natanzon.
United States Patent |
8,745,004 |
Natanzon , et al. |
June 3, 2014 |
Reverting an old snapshot on a production volume without a full
sweep
Abstract
It may be beneficial to revert from the production volume V to
the production snapshot S. Traditional approaches required a full
sweep of production volume data when reverting to a snapshot (i.e.,
reinitialize all data, mark all data as dirty and start replicating
to update the replication volume V' to what the production volume V
stores (i.e., the former production snapshot S). However, example
embodiments of the present invention provide for reverting from a
production volume to a snapshot without requiring a full sweep of
data in the production volume. Rather, example embodiments of the
present invention stop replication of the production volume, notify
a splitter of dirty location in the snapshot, synchronize the dirty
locations with the replication volume and resume replication to the
snapshot, thereby performing a minimal initialization.
Inventors: |
Natanzon; Assaf (Ramat Gan,
IL), Assouline; Benny (Rishon Letzion,
IL) |
Applicant: |
Name |
City |
State |
Country |
Type |
Natanzon; Assaf
Assouline; Benny |
Ramat Gan
Rishon Letzion |
N/A
N/A |
IL
IL |
|
|
Assignee: |
EMC Corporation (Hopkinton,
MA)
|
Family
ID: |
50781383 |
Appl.
No.: |
13/168,479 |
Filed: |
June 24, 2011 |
Current U.S.
Class: |
707/639; 707/640;
707/634 |
Current CPC
Class: |
G06F
11/1451 (20130101); G06F 16/10 (20190101); G06F
11/1469 (20130101); G06F 2201/84 (20130101) |
Current International
Class: |
G06F
17/30 (20060101) |
Field of
Search: |
;707/639,634,640 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Alam; Shahid
Attorney, Agent or Firm: Gupta; Krishnendu Kazanjian; Gerald
P.
Claims
What is claimed is:
1. A method for reverting from a production volume to a snapshot in
a replication environment comprising a production site including
the production volume, the snapshot, and a splitter, the method
comprising: stopping replication of the production volume; setting
a revert flag in the splitter; reverting the production volume to
the snapshot by notifying the splitter of dirty locations in the
snapshot and tracking metadata of the dirty locations in the
splitter; clearing the revert flag to notify the splitter that the
revert is completed; and resuming replication of the production
volume.
2. The method of claim 1 wherein the replication environment
further comprises a replication site including a replication volume
and wherein resuming replication of the production volume further
comprises: reading the metadata from the splitter; and
synchronizing the dirty locations with the replication volume.
3. The method of claim 1 wherein stopping replication of the
production volume comprises transitioning the splitter from a split
mode to a marking on host mode.
4. The method of claim 3 wherein resuming replication of the
production volume comprises: transitioning the splitter from the
marking on host mode to the split mode once the revert flag is
cleared; receiving dirty location metadata from the splitter for
each dirty location in the snapshot; and synchronizing the dirty
locations to the replication volume.
5. The method of claim 1 wherein reverting the production volume to
the snapshot comprises: moving the data of the snapshot to the
production volume; and deleting the snapshot data and the old
production volume data.
6. An apparatus for reverting from a production volume to a
snapshot in a replication environment comprising a production site
including the production volume, the snapshot, and a splitter, the
apparatus comprising: a controller configured to stop replication
of the production volume, set a revert flag in the splitter, clear
the revert flag to notify the splitter that the revert is
completed, and resume replication of the production volume after
reverting the production volume to the snapshot; and a manager
configured to revert the production volume to the snapshot by
notifying the splitter of dirty locations in the snapshot and
tracking metadata of the dirty locations in the splitter.
7. The apparatus of claim 6 wherein the replication environment
further comprises a replication site including a replication volume
and wherein the controller is further configured to read the
metadata from the splitter and synchronize the dirty locations with
the replication volume.
8. The apparatus of claim 6 wherein the controller is further
configured to transition the splitter from a split mode to a
marking on host mode.
9. The apparatus of claim 8 wherein the controller is further
configured to transition the splitter from the marking on host mode
to the split mode, once the revert flag is cleared, receive dirty
location metadata from the splitter for each dirty location in the
snapshot and synchronize the dirty locations to the replication
volume.
10. The apparatus of claim 6 wherein the manager is further
configured to move the data of the snapshot to the production
volume and delete the snapshot data and the old production volume
data.
11. A computer-program product including a non-transitory
computer-readable storage medium encoded with computer-program code
that, when executed on a processor of a computer, cause the
computer to revert from a production volume to a snapshot in a
replication environment comprising a production site including the
production volume, the snapshot, and a splitter, the
computer-program code comprising: computer-program code for
stopping replication of the production volume; computer-program
code for setting a revert flag in the splitter; computer-program
code for reverting the production volume to the snapshot notifying
the splitter of dirty locations in the snapshot and tracking
metadata of the dirty locations in the splitter; computer-program
code for clearing the revert flag to notify the splitter that the
revert is completed; and computer-program code for resuming
replication to the production volume.
12. The computer-program product of claim 11 wherein the
replication environment further comprises a replication site
including a replication volume and wherein computer-program code
for resuming replication to the production volume further
comprises: computer-program code for reading the metadata from the
splitter; and computer-program code for synchronizing the dirty
locations with the replication volume.
13. The computer-program product of claim 11 wherein
computer-program code for stopping replication of the production
volume comprises computer-program code for transitioning the
splitter from a split mode to a marking on host mode and wherein
computer-program code for resuming replication of the production
volume to the snapshot comprises: computer-program code for
transitioning the splitter from the marking on host mode to the
split mode once the revert flag is cleared; computer-program code
for receiving dirty location metadata from the splitter for each
dirty location in the snapshot; and synchronizing the dirty
locations to the replication volume.
14. The computer-program product of claim 11 wherein
computer-program code for reverting the production volume to the
snapshot comprises: computer-program code for moving the data of
the snapshot to the production volume; and computer-program code
for deleting the snapshot data and the old production volume data.
Description
A portion of the disclosure of this patent document may contain
command formats and other computer language listings, all of which
are subject to copyright protection. The copyright owner has no
objection to the facsimile reproduction by anyone of the patent
document or the patent disclosure, as it appears in the Patent and
Trademark Office patent file or records, but otherwise reserves all
copyright rights whatsoever.
TECHNICAL FIELD
This application relates to data replication.
BACKGROUND
Computer data is vital to today's organizations, and a significant
part of protection against disasters is focused on data protection.
As solid-state memory has advanced to the point where cost of
memory has become a relatively insignificant factor, organizations
can afford to operate with systems that store and process terabytes
of data.
Conventional data protection systems include tape backup drives,
for storing organizational production site data on a periodic
basis. Such systems suffer from several drawbacks. First, they
require a system shutdown during backup, since the data being
backed up cannot be used during the backup operation. Second, they
limit the points in time to which the production site can recover.
For example, if data is backed up on a daily basis, there may be
several hours of lost data in the event of a disaster. Third, the
data recovery process itself takes a long time.
Another conventional data protection system uses data replication,
by creating a copy of the organization's production site data on a
secondary backup storage system, and updating the backup with
changes. The backup storage system may be situated in the same
physical location as the production storage system, or in a
physically remote location. Data replication systems generally
operate either at the application level, at the file system level,
or at the data block level.
Current data protection systems try to provide continuous data
protection, which enable the organization to roll back to any
specified point in time within a recent history. Continuous data
protection systems aim to satisfy two conflicting objectives, as
best as possible; namely, (i) minimize the down time, in which the
organization production site data is unavailable, during a
recovery, and (ii) enable recovery as close as possible to any
specified point in time within a recent history.
Continuous data protection typically uses a technology referred to
as "journaling," whereby a log is kept of changes made to the
backup storage. During a recovery, the journal entries serve as
successive "undo" information, enabling rollback of the backup
storage to previous points in time. Journaling was first
implemented in database systems, and was later extended to broader
data protection.
SUMMARY
Example embodiments of the present invention provide a method, an
apparatus and a computer-program product for reverting from a
production volume to a snapshot. The method includes notifying a
splitter that a production volume is reverted to the snapshot. The
method also includes notifying the splitter that the revert is
completed and then resuming replicating to the snapshot.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and further advantages of the present invention may be
better under stood by referring to the following description taken
into conjunction with the accompanying drawings in which:
FIG. 1 is a simplified illustration of a data protection system, in
accordance with an embodiment of the present invention;
FIG. 2 is a simplified illustration of a write transaction for a
journal, in accordance with an embodiment of the present
invention;
FIG. 3 is a block diagram illustrating an example system for
reverting from a production volume to a snapshot according to an
example embodiment of the present invention;
FIGS. 4A-4D are flow diagrams illustrating respective example
methods for reverting from a production volume to a snapshot
according to an example embodiment of the present invention;
FIG. 5 is a block diagram of an example embodiment apparatus
according to the present invention; and
FIG. 6 is an illustration of an example embodiment of the present
invention as embodied in program code.
DETAILED DESCRIPTION
The following definitions are employed throughout the specification
and claims.
BACKUP SITE--a facility where replicated production site data is
stored; the backup site may be located in a remote site or at the
same location as the production site;
CLONE--a clone may be a copy or clone of the image or images, drive
or drives of a first location at a second location;
DELTA MARKING STREAM--may mean the tracking of the delta between
the production and replication site, which may contain the meta
data of changed locations, the delta marking stream may be kept
persistently on the journal at the production site of the
replication, based on the delta marking data the DPA knows which
locations are different between the production and the replica and
transfers them to the replica to make both sites identical;
DPA--a computer or a cluster of computers that serve as a data
protection appliance, responsible for data protection services
including inter alia data replication of a storage system, and
journaling of I/O requests issued by a host computer to the storage
system;
HOST--at least one computer or networks of computers that runs at
least one data processing application that issues I/O requests to
one or more storage systems; a host is an initiator with a SAN;
HOST DEVICE--an internal interface in a host, to a logical storage
unit;
IMAGE--a copy of a logical storage unit at a specific point in
time;
INITIATOR--a node in a SAN that issues I/O requests;
JOURNAL--a record of write transactions issued to a storage system;
used to maintain a duplicate storage system, and to rollback the
duplicate storage system to a previous point in time;
LOGICAL UNIT--a logical entity provided by a storage system for
accessing data from the storage system;
LUN--a logical unit number for identifying a logical unit;
PHYSICAL STORAGE UNIT--a physical entity, such as a disk or an
array of disks, for storing data in storage locations that can be
accessed by address;
PRODUCTION SITE--a facility where one or more host computers run
data processing applications that write data to a storage system
and read data from the storage system;
SAN--a storage area network of nodes that send and receive I/O and
other requests, each node in the network being an initiator or a
target, or both an initiator and a target;
SOURCE SIDE--a transmitter of data within a data replication
workflow, during normal operation a production site is the source
side; and during data recovery a backup site is the source
side;
SNAPSHOT--a Snapshot may refer to differential representations of
an image, i.e. the snapshot may have pointers to the original
volume, and may point to log volumes for changed locations.
Snapshots may be combined into a snapshot array, which may
represent different images over a time period;
SPLITTER/PROTECTION AGENT--may be an agent running either on a
production host a switch or a storage array which can intercept 10
and split them to a DPA and to the storage array, fail 10 redirect
10 or do any other manipulation to the IO;
STORAGE SYSTEM--a SAN entity that provides multiple logical units
for access by multiple SAN initiators;
TARGET--a node in a SAN that replies to I/O requests;
TARGET SIDE--a receiver of data within a data replication workflow;
during normal operation a back site is the target side, and during
data recovery a production site is the target side; and
WAN--a wide area network that connects local networks and enables
them to communicate with one another, such as the Internet.
Description of Embodiments Using a Five State Journaling
Process
FIG. 1 is a simplified illustration of a data protection system
100, in accordance with an embodiment of the present invention.
Shown in FIG. 1 are two sites; Site I, which is a production site,
on the right, and Site II, which is a backup site, on the left.
Under normal operation the production site is the source side of
system 100, and the backup site is the target side of the system.
The backup site is responsible for replicating production site
data. Additionally, the backup site enables rollback of Site I data
to an earlier pointing time, which may be used in the event of data
corruption of a disaster, or alternatively in order to view or to
access data from an earlier point in time.
During normal operations, the direction of replicate data flow goes
from source side to target side. It is possible, however, for a
user to reverse the direction of replicate data flow, in which case
Site I starts to behave as a target backup site, and Site II starts
to behave as a source production site. Such change of replication
direction is referred to as a "failover". A failover may be
performed in the event of a disaster at the production site, or for
other reasons. In some data architectures, Site I or Site II
behaves as a production site for a portion of stored data, and
behaves simultaneously as a backup site for another portion of
stored data. In some data architectures, a portion of stored data
is replicated to a backup site, and another portion is not.
The production site and the backup site may be remote from one
another, or they may both be situated at a common site, local to
one another. Local data protection has the advantage of minimizing
data lag between target and source, and remote data protection has
the advantage is being robust in the event that a disaster occurs
at the source side.
The source and target sides communicate via a wide area network
(WAN) 128, although other types of networks are also adaptable for
use with the present invention.
In accordance with an embodiment of the present invention, each
side of system 100 includes three major components coupled via a
storage area network (SAN); namely, (i) a storage system, (ii) a
host computer, and (iii) a data protection appliance (DPA).
Specifically with reference to FIG. 1, the source side SAN includes
a source host computer 104, a source storage system 108, and a
source DPA 112. Similarly, the target side SAN includes a target
host computer 116, a target storage system 120, and a target DPA
124.
Generally, a SAN includes one or more devices, referred to as
"nodes". A node in a SAN may be an "initiator" or a "target", or
both. An initiator node is a device that is able to initiate
requests to one or more other devices; and a target node is a
device that is able to reply to requests, such as SCSI commands,
sent by an initiator node. A SAN may also include network switches,
such as fiber channel switches. The communication links between
each host computer and its corresponding storage system may be any
appropriate medium suitable for data transfer, such as fiber
communication channel links.
In an embodiment of the present invention, the host communicates
with its corresponding storage system using small computer system
interface (SCSI) commands.
System 100 includes source storage system 108 and target storage
system 120. Each storage system includes physical storage units for
storing data, such as disks or arrays of disks. Typically, storage
systems 108 and 120 are target nodes. In order to enable initiators
to send requests to storage system 108, storage system 108 exposes
one or more logical units (LU) to which commands are issued. Thus,
storage systems 108 and 120 are SAN entities that provide multiple
logical units for access by multiple SAN initiators.
Logical units are a logical entity provided by a storage system,
for accessing data stored in the storage system. A logical unit is
identified by a unique logical unit number (LUN). In an embodiment
of the present invention, storage system 108 exposes a logical unit
136, designated as LU A, and storage system 120 exposes a logical
unit 156, designated as LU B.
In an embodiment of the present invention, LU B is used for
replicating LU A. As such, LU B is generated as a copy of LU A. In
one embodiment, LU B is configured so that its size is identical to
the size of LU A. Thus for LU A, storage system 120 serves as a
backup for source side storage system 108. Alternatively, as
mentioned hereinabove, some logical units of storage system 120 may
be used to back up logical units of storage system 108, and other
logical units of storage system 120 may be used for other purposes.
Moreover, in certain embodiments of the present invention, there is
symmetric replication whereby some logical units of storage system
108 are used for replicating logical units of storage system 120,
and other logical units of storage system 120 are used for
replicating other logical units of storage system 108.
System 100 includes a source side host computer 104 and a target
side host computer 116. A host computer may be one computer, or a
plurality of computers, or a network of distributed computers, each
computer may include inter alia a conventional CPU, volatile and
non-volatile memory, a data bus, an I/O interface, a display
interface and a network interface. Generally a host computer runs
at least one data processing application, such as a database
application and an e-mail server.
Generally, an operating system of a host computer creates a host
device for each logical unit exposed by a storage system in the
host computer SAN. A host device is a logical entity in a host
computer, through which a host computer may access a logical unit.
In an embodiment of the present invention, host device 104
identifies LU A and generates a corresponding host device 140,
designated as Device A, through which it can access LU A.
Similarly, host computer 116 identifies LU B and generates a
corresponding device 160, designated as Device B.
In an embodiment of the present invention, in the course of
continuous operation, host computer 104 is a SAN initiator that
issues I/O requests (write/read operations) through host device 140
to LU A using, for example, SCSI commands. Such requests are
generally transmitted to LU A with an address that includes a
specific device identifier, an offset within the device, and a data
size. Offsets are generally aligned to 512 byte blocks. The average
size of a write operation issued by host computer 104 may be, for
example, 10 kilobytes (KB); i.e., 20 blocks. For an I/O rate of 50
megabytes (MB) per second, this corresponds to approximately 5,000
write transactions per second.
System 100 includes two data protection appliances, a source side
DPA 112 and a target side DPA 124. A DPA performs various data
protection services, such as data replication of a storage system,
and journaling of I/O requests issued by a host computer to source
side storage system data. As explained in detail hereinbelow, when
acting as a target side DPA, a DPA may also enable rollback of data
to an earlier point in time, and processing of rolled back data at
the target site. Each DPA 112 and 124 is a computer that includes
inter alia one or more conventional CPUs and internal memory.
For additional safety precaution, each DPA is a cluster of such
computers. Use of a cluster ensures that if a DPA computer is down,
then the DPA functionality switches over to another computer. The
DPA computers within a DPA cluster communicate with one another
using at least one communication link suitable for data transfer
via fiber channel or IP based protocols, or such other transfer
protocol. One computer from the DPA cluster serves as the DPA
leader. The DPA cluster leader coordinates between the computers in
the cluster, and may also perform other tasks that require
coordination between the computers, such as load balancing.
In the architecture illustrated in FIG. 1, DPA 112 and DPA 124 are
standalone devices integrated within a SAN. Alternatively, each of
DPA 112 and DPA 124 may be integrated into storage system 108 and
storage system 120, respectively, or integrated into host computer
104 and host computer 116, respectively. Both DPAs communicate with
their respective host computers through communication lines such as
fiber channels using, for example, SCSI commands.
In accordance with an embodiment of the present invention, DPAs 112
and 124 are configured to act as initiators in the SAN; i.e., they
can issue I/O requests using, for example, SCSI commands, to access
logical units on their respective storage systems. DPA 112 and DPA
124 are also configured with the necessary functionality to act as
targets; i.e., to reply to I/O requests, such as SCSI commands,
issued by other initiators in the SAN, including inter alia their
respective host computers 104 and 116. Being target nodes, DPA 112
and DPA 124 may dynamically expose or remove one or more logical
units.
As described hereinabove, Site I and Site II may each behave
simultaneously as a production site and a backup site for different
logical units. As such, DPA 112 and DPA 124 may each behave as a
source DPA for some logical units, and as a target DPA for other
logical units, at the same time.
In accordance with an embodiment of the present invention, host
computer 104 and host computer 116 include protection agents 144
and 164, respectively. Protection agents 144 and 164 intercept SCSI
commands issued by their respective host computers, via host
devices to logical units that are accessible to the host computers.
In accordance with an embodiment of the present invention, a data
protection agent may act on an intercepted SCSI commands issued to
a logical unit, in one of the following ways: Send the SCSI
commands to its intended logical unit; Redirect the SCSI command to
another logical unit; Split the SCSI command by sending it first to
the respective DPA. After the DPA returns an acknowledgement, send
the SCSI command to its intended logical unit; Fail a SCSI command
by returning an error return code; and Delay a SCSI command by not
returning an acknowledgement to the respective host computer.
A protection agent may handle different SCSI commands, differently,
according to the type of the command. For example, a SCSI command
inquiring about the size of a certain logical unit may be sent
directly to that logical unit, while a SCSI write command may be
split and sent first to a DPA associated with the agent. A
protection agent may also change its behavior for handling SCSI
commands, for example as a result of an instruction received from
the DPA.
Specifically, the behavior of a protection agent for a certain host
device generally corresponds to the behavior of its associated DPA
with respect to the logical unit of the host device. When a DPA
behaves as a source site DPA for a certain logical unit, then
during normal course of operation, the associated protection agent
splits I/O requests issued by a host computer to the host device
corresponding to that logical unit. Similarly, when a DPA behaves
as a target device for a certain logical unit, then during normal
course of operation, the associated protection agent fails I/O
requests issued by host computer to the host device corresponding
to that logical unit.
Communication between protection agents and their respective DPAs
may use any protocol suitable for data transfer within a SAN, such
as fiber channel, or SCSI over fiber channel. The communication may
be direct, or via a logical unit exposed by the DPA. In an
embodiment of the present invention, protection agents communicate
with their respective DPAs by sending SCSI commands over fiber
channel.
In an embodiment of the present invention, protection agents 144
and 164 are drivers located in their respective host computers 104
and 116. Alternatively, a protection agent may also be located in a
fiber channel switch, or in any other device situated in a data
path between a host computer and a storage system.
What follows is a detailed description of system behavior under
normal production mode, and under recovery mode.
In accordance with an embodiment of the present invention, in
production mode DPA 112 acts as a source site DPA for LU A. Thus,
protection agent 144 is configured to act as a source side
protection agent; i.e., as a splitter for host device A.
Specifically, protection agent 144 replicates SCSI I/O requests. A
replicated SCSI I/O request is sent to DPA 112. After receiving an
acknowledgement from DPA 124, protection agent 144 then sends the
SCSI I/O request to LU A. Only after receiving a second
acknowledgement from storage system 108 will host computer 104
initiate another I/O request.
When DPA 112 receives a replicated SCSI write request from data
protection agent 144, DPA 112 transmits certain I/O information
characterizing the write request, packaged as a "write
transaction", over WAN 128 to DPA 124 on the target side, for
journaling and for incorporation within target storage system
120.
DPA 112 may send its write transactions to DPA 124 using a variety
of modes of transmission, including inter alia (i) a synchronous
mode, (ii) an asynchronous mode, and (iii) a snapshot mode. In
synchronous mode, DPA 112 sends each write transaction to DPA 124,
receives back an acknowledgement from DPA 124, and in turns sends
an acknowledgement back to protection agent 144. Protection agent
144 waits until receipt of such acknowledgement before sending the
SCSI write request to LU A.
In asynchronous mode, DPA 112 sends an acknowledgement to
protection agent 144 upon receipt of each I/O request, before
receiving an acknowledgement back from DPA 124.
In snapshot mode, DPA 112 receives several I/O requests and
combines them into an aggregate "snapshot" of all write activity
performed in the multiple I/O requests, and sends the snapshot to
DPA 124, for journaling and for incorporation in target storage
system 120. In snapshot mode DPA 112 also sends an acknowledgement
to protection agent 144 upon receipt of each I/O request, before
receiving an acknowledgement back from DPA 124.
For the sake of clarity, the ensuing discussion assumes that
information is transmitted at write-by-write granularity.
While in production mode, DPA 124 receives replicated data of LU A
from DPA 112, and performs journaling and writing to storage system
120. When applying write operations to storage system 120, DPA 124
acts as an initiator, and sends SCSI commands to LU B.
During a recovery mode, DPA 124 undoes the write transactions in
the journal, so as to restore storage system 120 to the state it
was at, at an earlier time.
As described hereinabove, in accordance with an embodiment of the
present invention, LU B is used as a backup of LU A. As such,
during normal production mode, while data written to LU A by host
computer 104 is replicated from LU A to LU B, host computer 116
should not be sending I/O requests to LU B. To prevent such I/O
requests from being sent, protection agent 164 acts as a target
site protection agent for host Device B and fails I/O requests sent
from host computer 116 to LU B through host Device B.
In accordance with an embodiment of the present invention, target
storage system 120 exposes a logical unit 176, referred to as a
"journal LU", for maintaining a history of write transactions made
to LU B, referred to as a "journal". Alternatively, journal LU 176
may be striped over several logical units, or may reside within all
of or a portion of another logical unit. DPA 124 includes a journal
processor 180 for managing the journal.
Journal processor 180 functions generally to manage the journal
entries of LU B. Specifically, journal processor 180 (i) enters
write transactions received by DPA 124 from DPA 112 into the
journal, by writing them into the journal LU, (ii) applies the
journal transactions to LU B, and (iii) updates the journal entries
in the journal LU with undo information and removes already-applied
transactions from the journal. As described below, with reference
to FIGS. 2 and 3A-3D, journal entries include four streams, two of
which are written when write transaction are entered into the
journal, and two of which are written when write transaction are
applied and removed from the journal.
FIG. 2 is a simplified illustration of a write transaction 200 for
a journal, in accordance with an embodiment of the present
invention. The journal may be used to provide an adaptor for access
to storage 120 at the state it was in at any specified point in
time. Since the journal contains the "undo" information necessary
to rollback storage system 120, data that was stored in specific
memory locations at the specified point in time may be obtained by
undoing write transactions that occurred subsequent to such point
in time.
Write transaction 200 generally includes the following fields: one
or more identifiers; a time stamp, which is the date & time at
which the transaction was received by source side DPA 112; a write
size, which is the size of the data block; a location in journal LU
176 where the data is entered; a location in LU B where the data is
to be written; and the data itself.
Write transaction 200 is transmitted from source side DPA 112 to
target side DPA 124. As shown in FIG. 2, DPA 124 records the write
transaction 200 in four streams. A first stream, referred to as a
DO stream, includes new data for writing in LU B. A second stream,
referred to as an DO METADATA stream, includes metadata for the
write transaction, such as an identifier, a date & time, a
write size, a beginning address in LU B for writing the new data
in, and a pointer to the offset in the do stream where the
corresponding data is located. Similarly, a third stream, referred
to as an UNDO stream, includes old data that was overwritten in LU
B; and a fourth stream, referred to as an UNDO METADATA, include an
identifier, a date & time, a write size, a beginning address in
LU B where data was to be overwritten, and a pointer to the offset
in the undo stream where the corresponding old data is located.
In practice each of the four streams holds a plurality of write
transaction data. As write transactions are received dynamically by
target DPA 124, they are recorded at the end of the DO stream and
the end of the DO METADATA stream, prior to committing the
transaction. During transaction application, when the various write
transactions are applied to LU B, prior to writing the new DO data
into addresses within the storage system, the older data currently
located in such addresses is recorded into the UNDO stream.
By recording old data, a journal entry can be used to "undo" a
write transaction. To undo a transaction, old data is read from the
UNDO stream in a reverse order, from the most recent data to the
oldest data, for writing into addresses within LU B. Prior to
writing the UNDO data into these addresses, the newer data residing
in such addresses is recorded in the DO stream.
The journal LU is partitioned into segments with a pre-defined
size, such as 1 MB segments, with each segment identified by a
counter. The collection of such segments forms a segment pool for
the four journaling streams described hereinabove. Each such stream
is structured as an ordered list of segments, into which the stream
data is written, and includes two pointers--a beginning pointer
that points to the first segment in the list and an end pointer
that points to the last segment in the list.
According to a write direction for each stream, write transaction
data is appended to the stream either at the end, for a forward
direction, or at the beginning, for a backward direction. As each
write transaction is received by DPA 124, its size is checked to
determine if it can fit within available segments. If not, then one
or more segments are chosen from the segment pool and appended to
the stream's ordered list of segments.
Thereafter the DO data is written into the DO stream, and the
pointer to the appropriate first or last segment is updated.
Freeing of segments in the ordered list is performed by simply
changing the beginning or the end pointer. Freed segments are
returned to the segment pool for re-use.
A journal may be made of any number of streams including less than
or more than 5 streams. Often, based on the speed of the journaling
and whether the back-up is synchronous or a synchronous a fewer or
greater number of streams may be used.
Delta Marking
A delta marker stream may contain the locations that may be
different between the latest I/O data which arrived to the remote
side (the current remote site) and the latest I/O data which
arrived at the local side. In particular, the delta marking stream
may include metadata of the differences between the source side and
the target side. For example, every I/O reaching the data
protection appliance for the source 112 may be written to the delta
marking stream and data is freed from the delta marking stream when
the data safely arrives at both the source volume of replication
108 and the remote journal 180 (e.g. DO stream). Specifically,
during an initialization process no data may be freed from the
delta marking stream; and only when the initialization process is
completed and I/O data has arrived to both local storage and the
remote journal data, may be I/O data from the delta marking stream
freed. When the source and target are not synchronized, data may
not be freed from the delta marking stream. The initialization
process may start by merging delta marking streams of the target
and the source so that the delta marking stream includes a list of
all different locations between local and remote sites. For
example, a delta marking stream at the target might have data too
if a user has accessed an image at the target site.
The initialization process may create one virtual disk out of all
the available user volumes. The virtual space may be divided into a
selected number of portions depending upon the amount of data
needed to be synchronized. A list of `dirty` blocks may be read
from the delta marker stream that is relevant to the area currently
being synchronized to enable creation of a dirty location data
structure. The system may begin synchronizing units of data, where
a unit of data is a constant amount of dirty data, e.g., a data
that needs to be synchronized.
The dirty location data structure may provide a list of dirty
location until the amount of dirty location is equal to the unit
size or until there is no data left. The system may begin a
so-called ping pong process to synchronize the data. The process
may transfer the differences between the production and replica
site to the replica.
A discussion of mirroring may be found in U.S. Pat. No. 7,346,805,
entitled "PROTECTION OF MIRRORED DATA," issued on Mar. 18, 2008 and
assigned to EMC Corporation of Hopkinton, Mass., which is hereby
incorporated by reference in its entirety.
A discussion of journaling and some techniques associated with
journaling may be found in U.S. Pat. No. 7,516,287, entitled
"METHODS AND APPARATUS FOR OPTIMAL JOURNALING FOR CONTINUOUS DATA
REPLICATION," issued on Apr. 7, 2009 and assigned to EMC
Corporation of Hopkinton, Mass., which is hereby incorporated by
reference in its entirety.
A discussion of dynamically adding storage for a journal may be
found in U.S. Pat. No. 7,840,536, entitled "METHODS AND APPARATUS
FOR DYNAMIC JOURNAL EXPANSION," issued on Nov. 23, 2010 and
assigned to EMC Corporation of Hopkinton, Mass., which is hereby
incorporated by reference in its entirety.
Reverting an Old Snapshot on a Production Volume without a Full
Sweep
FIG. 3 is a block diagram of a replication environment 300
including a production site and a replication site. The production
site includes storage 308 (e.g., a storage array) configured to
receive I/O commands from a host 304 via a switch 348. A splitter
310 in the production site is configured to intercept the I/O
commands from the host 304 and split them to both a user volume 336
at the production site and a data protection appliance (DPA) 312.
Replication of the I/Os then occurs from the production site DPA
312 to the replications site DPA 324 to be maintained in the
replications site journal 376. As described above, the production
site DPA 312 performs replications across the WAN 328 to the
replication volume 356 via the replication site DPA 324.
Replication also may be to another device at the production site;
in that case the production site DPA 312 behaves also as the
replication site DPA 324). The replication volume 356 and a
replication site journal 376 may be stored in replication site
storage 320, such as a storage array. The replications site may
also include a host 316 and a switch 368.
In certain scenarios, it may be beneficial to revert from the
production volume V to the production snapshot S. For example, if
the production volume V becomes corrupted, it would be advantageous
to be able to revert to the snapshot S. Traditionally, former
approaches required a full sweep of production volume data when
reverting to a snapshot (i.e., reinitialize all data, mark all data
as dirty and start replicating to update the replication volume V'
to what the production volume V stores (i.e., the former production
snapshot S).
However, example embodiments of the present invention provide for
reverting from a production volume to a snapshot in a replication
environment comprising a production site including the production
volume and a splitter and a replication site including a
replication volume. Example embodiments of the present invention do
not require a full sweep of data in the production volume, as is
traditionally required. Rather, example embodiments of the present
invention perform a minimal initialization of locations in the
production volume (i.e., the reverted snapshot) which are dirty
(i.e., the locations in the reverted snapshot S which are different
in than the volume V).
The description below is intended to be read with respect to the
block diagram of FIG. 3 in conjunction with the flow diagram of
FIGS. 4A-4D illustrating embodiment methods for reverting from a
production volume to a snapshot. Reverting from the production
volume V to the production snapshot S is transparent at the
production site because the transition occurs immediately. However,
the revert is not transparent at the replication site because the
replication connection between the production volume V and
replication volume V' is broken in transitioning from the
production volume V to the production snapshot S, where the
snapshot S is at a different point in time than the replication
volume V'.
Accordingly, in an example embodiment of the present invention
illustrated in FIGS. 3 and 4A, the storage 308 notifies the
splitter 310 that the production volume V requires reverting to the
snapshot S, for example, if the production volume V has become
corrupted, or for any other reason. The splitter 310 then stops
replication of the production volume V (410). For example, when the
storage 308 notifies the splitter 310 of the revert, the splitter
310 stops replication of the production volume V for a short period
of time to avoid an inconsistent point in time. The revert may be
reflected in the splitter 310 by the storage 308 setting a revert
flag in the splitter 310.
The splitter 310 then reverts the production volume V to the
snapshot S (420), typically as a result of a user action, after
which the storage 308 will automatically notify the splitter 310 of
the revert. As illustrated in FIG. 4B, to revert the production
volume V to the snapshot S, the storage will consider the content
of the volume V will to now be the content of the snapshot S (421),
and the storage 308 will delete the production volume snapshot S as
well as the old data of the production volume V (422) (i.e., there
is no change in configuration; rather the production volume V
remains with the same identity but gets the data from the snapshot
S).
Returning to FIG. 4A, once the revert is completed, replication of
the production volume (430) between the production site and the
replication site according to the production site DPA 312 and the
replication site DPA 324 may then resume. As described above, the
snapshot S at this point has been erased and the volume V contains
the data which was once the data of snapshot S. It should be noted
that, for the revert, the host 304 is usually down (i.e., the host
is shut down and does not generate any new I/O commands) because
reverting from the production volume V to the snapshot S changes
its underlying data.
In a further example embodiment illustrated in the flow diagram of
FIG. 4C, before the storage 308 can complete reverting the snapshot
S, to resume replication of the production volume V (e.g., step 430
of FIG. 4A), the storage 308 may notify the splitter 310 of dirty
locations in the snapshot S (i.e., the locations which changed in
volume V during the revert process) (431). Metadata of those dirty
locations then may be tracked in the splitter 310 (432) and read
from the splitter by the production site DPA 312 (433) to be
synchronized with the replication volume V' (434). Likewise, to
notify the splitter 310 that the revert is complete (e.g., step 420
of FIG. 4A), the storage 308 may take down the revert flag.
In another example embodiment, as illustrated in the flow diagram
of FIG. 4D, when notifying the splitter that the production volume
requires reverting to the snapshot, the storage 308 may cause the
splitter 310 to transition into Marking on Host mode (436). In
Marking on Host mode, the splitter 310 stops sending I/Os to the
production volume V and the production site DPA 312. Rather, the
I/Os are tracked in a bitmap or a metadata list in the splitter 310
instead of being sent to the storage 308 or the DPA 312. This stops
replication immediately without losing any I/Os.
Additionally, the DPA 312 periodically queries the splitter 310 for
its state. Accordingly, the DPA 312 discovers that the splitter 310
is in Marking on Host mode and the revert flag is set, according to
the state returned by the splitter 310 from the DPA's 312 query.
The DPA 312 then stops replication to the replication site. The
storage 308 notifies splitter 310 of dirty locations in the
snapshot S, which are tracked in memory by the splitter 310 (e.g.,
step 431 of FIG. 4C). For example, the storage 308 may notify the
splitter 310 of a particular offset range that contains changes.
Once the storage 308 completes notifying the splitter 310 of the
dirty locations, the storage 308 notifies the splitter 310 that the
revert is complete.
With the splitter 310 notified of dirty locations in the snapshot S
for synchronization to the replication volume V', the revert is
complete and the splitter 310 is notified (e.g., step 420 of FIG.
4A), such as by lowering the revert flag. Replication may now
resume. The DPA 312, which periodically queries the splitter 310
for its state, then determines that the revert is complete and
reads the dirty location metadata from the splitter 310 (437). For
example, the DPA 312 may read the Marking on Host data into a delta
marking stream in the production site journal 384 and begin
replication by resynchronizing the dirty locations to the
replication volume V' (438). The delta marking stream provides a
greater amount of memory for the changes than the splitter and
therefore is able to more accurately store the bitmaps and lists of
changes. The storage 308 may then transition the splitter 310 from
Marking on Host mode back to split mode (439) and replication may
resume.
FIG. 5 is a block diagram of an example embodiment DPA 535
according to the present invention. The DPA includes memory 590
storing program logic for reverting from a production volume to a
snapshot, a processor 580, a communications interface 560, a
manager 581 and a controller 582.
The methods and apparatus of this invention may take the form, at
least partially, of program code (i.e., instructions) embodied in
tangible non-transitory media, such as floppy diskettes, CD-ROMs,
hard drives, random access or read only-memory, or any other
machine-readable storage medium. When the program code is loaded
into and executed by a machine, such as the computer of FIG. 5, the
machine becomes an apparatus for practicing the invention. When
implemented on one or more general-purpose processors, the program
code combines with such a processor to provide a unique apparatus
that operates analogously to specific logic circuits. As such a
general purpose digital machine can be transformed into a special
purpose digital machine.
FIG. 6 shows program logic 655 embodied on a computer-readable
medium 660 as shown, and wherein the logic is encoded in
computer-executable code configured for carrying out the methods
for reverting from a production volume to a snapshot of this
invention and thereby forming a computer program product 600.
The logic for carrying out the method may be embodied as part of
the aforementioned system, which is useful for carrying out a
method described with reference to embodiments shown in, for
example, FIGS. 3 and 4A-4D. For purposes of illustrating the
present invention, the invention is described as embodied in a
specific configuration and using special logical arrangements, but
one skilled in the art will appreciate that the device is not
limited to the specific configuration but rather only by the claims
included with this specification.
Although the foregoing invention has been described in some detail
for purposes of clarity of understanding, it will be apparent that
certain changes and modifications may be practiced within the scope
of the appended claims. Accordingly, the present implementations
are to be considered as illustrative and not restrictive, and the
invention is not to be limited to the details given herein, but may
be modified within the scope and equivalents of the appended
claims.
In reading the above description, persons skilled in the art will
realize that there are many apparent variations that can be applied
to the methods and systems described. In the foregoing
specification, the invention has been described with reference to
specific exemplary embodiments thereof. It will, however, be
evident that various modifications and changes may be made to the
specific exemplary embodiments without departing from the broader
spirit and scope of the invention as set forth in the appended
claims. Accordingly, the specification and drawings are to be
regarded in an illustrative rather than a restrictive sense.
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